Abstract 3467: A novel interplay between Rap1, Epac and PKA regulates induction of angiogenesis

Abstract

Abstract Advanced stage prostate cancer is associated with poor prognosis since therapeutic options are limited when the disease has progressed into a hormone refractory state. One therapeutic strategy is angiogenesis inhibition, which interferes with the blood supply of the tumor instead of targeting tumor cells directly. Here we focus on the role of three factors as regulators of angiogenesis: Rap1, a small GTPase homologous to Ras; Epac, a cAMP-activated guanine nucleotide factor for Rap1; and Protein kinase A (PKA), a cAMP-activated kinase. In previous studies, we showed that activation of Epac/Rap1 in human microvascular endothelial cells (HMVECs) inhibited VEGF-mediated chemotaxis and angiogenesis in a SCID mouse model of human angiogenesis. To test the efficacy of Epac/Rap1 activation in a prostate tumor xenograft model, we analyzed the effects of constitutive Rap1 or 8 CPT-2Me-cAMP (8CPT), a strong Epac agonist and weak PKA agonist, in PC-3 cells. PC3 cells rank high in expression levels of Rap1. Surprisingly, although 8CPT did not alter tumor growth in PC-3 cells, the drug significantly reduced tumor size in PC-3 cells expressing constitutively active Rap1. Tumor analysis showed an increase in VEGF and CD31 immunostaining in Rap1-expressing tumors, and a significant decrease in their levels upon treatment of the mice with 8CPT. To understand the mechanism of tumor inhibition, we cultured PC-3 cells under hypoxic conditions that mimic the tumor microenvironment. As observed in vivo, constitutive Rap-1 expression induced VEGF and Glut-1 mRNA levels as well as HIF-1α protein, and 8CPT reversed this effect. However, in contrast to HMVECs, 8CPT acts in PC-3 cells via PKA. A PKA activator mimicked 8CPT by inhibiting VEGF secretion in PC3-Rap1 cells under hypoxic conditions. Furthermore, the PKA inhibitor, H-89, reversed the inhibition of VEGF secretion in PC3-Rap1 cells in culture and inhibited growth in mouse xenografts. Thus, expression of Rap1 in PC-3 human prostate tumor cells promotes hypoxic induction of angiogenesis and increases the susceptibility of the cells to PKA inhibition. Taken together, our results suggest that a novel interplay between Rap1, Epac and PKA regulates tumor-microenvironment induction of angiogenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3467. doi:10.1158/1538-7445.AM2011-3467